Porous silicon was found by A. Uhlir and D. R. Turner who were studying electropolishing of single-crystal silicon biased to a positive potential in an aqueous solution of hydrofluoric acid.
Later, to exploit excellent reactivity of porous silicon, application of porous silicon to the element isolation process in manufacturing a silicon integrated circuit was examined, and a full isolation technology (FIFOS: Full Isolation by Porous Oxidized Silicon) using a porous silicon oxide film was developed (K. Imai, Solid State Electron 24, 159, 1981).
Recently, an applied technology to direct bonding has been developed in which a silicon epitaxial layer is grown on a porous silicon substrate, and the substrate is bonded to an amorphous substrate or single-crystal silicon substrate via the oxide film (Japanese Patent Laid-Open No. 5-21338).
As another application example, porous silicon has received a great deal of attention as a photoluminescence or electroluminescence material that emits light by itself (Japanese Patent Laid-Open No. 6-338631).
A conventional anodizing apparatus for manufacturing a substrate having a porous silicon layer will be described below.
FIG. 20 is a view showing the arrangement of a conventional anodizing apparatus (Japanese Patent Laid-Open No. 60-94737). In this anodizing apparatus, anodizing tanks 1902a and 1902b made of Teflon (tradename of du Pont in the U.S.A) as a material with HF resistance are arranged to sandwich a silicon substrate 1901 from both sides. The anodizing tanks 1902a and 1902b respectively have O-rings 1904a and 1904b for sealing at portions where the silicon substrate 1901 is held. The anodizing tanks 1902a and 1902b have platinum electrodes 1903a and 1903b, respectively. After the silicon substrate 1901 is sandwiched by the two anodizing tanks 1902a and 1902b, the anodizing tanks 1902a and 1902b are filled with HF solutions 1905a and 1905b, respectively. In this state, a DC voltage is applied between the electrodes by setting the platinum electrode 1903a as a negative electrode and the platinum electrode 1903b as a positive electrode. The silicon substrate 1901 is anodized, and a porous silicon layer is formed on the negative-electrode-side surface of the silicon substrate 1901.
In such conventional scheme of vertically holding a silicon substrate and anodizing it, a gas (e.g., hydrogen gas) generated by the anodizing reaction may stay on the surface of the silicon substrate for a long time or rise along the surface of the silicon substrate. In this case, the track of gas remains on the surface of the porous layer formed on the silicon substrate. This makes the porous layer nonuniform to result in poor quality and a decrease in yield and productivity. Hence, a demand has arisen for introduction of a new scheme of preventing a gas generated by the anodizing reaction from adversely affecting anodizing.
To obtain high quality and productivity for substrates having a porous silicon layer, it is important to reduce contamination of a silicon substrate during anodizing, and reduce contamination of a silicon substrate during a series of processes including anodizing and associated processes (e.g., washing and drying).
To increase productivity of substrates having a porous silicon layer, it is also important to increase the speed of the series of processes including anodizing and associated processes.
Additionally, in consideration of the recent tendency of an increase in diameter of silicon substrates, it is also important to propose a scheme capable of easily coping with the increase in diameter.